Heretofore, heat-sealed packaging materials have been widely used in many fields including food packaging, and in particular, those which can be peeled at the heat-seal interface have been widely used as easily openable or peelable heat-sealed closures.
In a peelable container, sealing characteristics and opening characteristics are in a contradictory relation, and when the sealing characteristics are such as to withstand the increasing of the internal pressure at the time of retorting sterilization, its opening characteristics will be impaired. For example, in Japan, heat-sealed packages containing retorted goods should have a seal strength at the heat-sealed part of at least 2.3 kg/15 mm width. When the seal strength between a container body and a closure reaches this value, it is difficult to perform hand peeling on the heat-sealed interface between the two.
As a solution to this problem, Plastics, Vol. 38, No., 5, page 65 (1987) already proposed a heat-sealed container in which a heat-sealable inner surface material layer is provided throughout its entire inner surface, the inner material layer and a layer located inwardly thereof are sealed for easy peeling, the inner material layer and a closure are sealed tightly, and a score for cutting of the inner surface layer is provided at a site more toward the center than the heat seal portion whereby opening from outside is rendered easy while the heat seal strength outwardly from the center is maintained at 2.3 kg/15 mm or higher.
The above-mentioned easily openable heat-sealed container is very significant in that easy openability is imparted to the package while the seal strength is maintained at 2.3 kg/15 mm or higher. But the structure or production of the container is limited because, for example, the container must be subjected to score formation. Furthermore, to form a score inwardly of its heat sealing peripheral portion, the material of which the container is made is restricted. For example, a laminated container including an interlayer of a metallic foil such as a steel foil becomes useless because of rust formation at the score portion. Moreover, to provide the score, the area of the flange portion of the container naturally becomes large; consequently, the amount of the container material used and the bulk of the package increase and the appearance characteristics of the container become undesirable.
Generally, it is never the case with easily openable heat-sealed containers that they can be easily opened if simply an easily peelable seal is provided in the heat-sealed portion. To start opening easily, it is necessary to design the container such that stresses will be concentrated on an opening start part of the easily peelable seal portion.